I have a 70 Mach 1 that I have had for 15 years. Finally getting around to a major overhaul. Question on the rear end. Over the years it no longer will engage both wheels, I believe it is limited slip. What would cause it to only spin one wheel now? Do I have to rebuild the rearend or do I just need a fluid fill? The rear end has never been serviced and has 130,000 miles on it (some are hard miles from my high school days) The axle code is 6. I'm pretty sure this would be a Ford 9 inch. Correct me if I'm wrong. I would like to put a steeper gear in it. What is the best (economical) way to get both tires engaging and have a steeper gear (3.75?) Thanks for all your help